1. Field of the Invention
This invention relates generally to servo control systems and, more particularly, to disk drive servo control systems for control of disk arm assembly movement across the surface of a rotating disk.
2. Description of the Related Art
In conventional computer data storage systems having a rotating storage medium, such as a rotating magnetic or magneto-optical disk, data is stored in a series of concentric or spiral tracks across the surface of the disk. The data comprises a series of variations in disk surface magnetic orientation in the tracks. The variations in magnetic orientation, generally comprising reversals of magnetic flux, represent binary digits of ones and zeroes that in turn represent data. The binary digits must be read from the disk surface by a magnetic read/write head suspended over the disk surface that can detect the variations in magnetic orientation as the disk rotates relative to the read/write head at thousands of revolutions per minute.
Reading data from a desired one of the tracks on the disk surface requires knowledge of the read/write head position relative to the track as the disk rotates and the head is moved across the disk and requires precise centering of the head over the disk track. Conventionally, the read/write head is mounted on a disk arm that is moved by a servo. A disk drive servo control system controls movement of the arm across the surface of the disk to move the read/write head from track to track and, once over a selected track, to maintain the read/write head in a path over the centerline of the track. Maintaining the read/write head centered over a track permits accurate reading and recording of data in the track.
A servo control system maintains the read/write head centered over a track by reading servo information from the disk surface. The servo information comprises a servo pattern of high frequency magnetic flux transitions, generally flux reversals, that are prerecorded in the tracks. A servo read head, which can be the same head used for reading the binary data or can be a dedicated servo pattern head, detects the servo pattern and produces an analog signal. The servo pattern analog signal is demodulated by servo control system circuitry to provide information on the track from which the servo pattern was read and on the position of the read/write head relative to the track and also to produce a position error signal that is used to control the disk arm servo. In this way, the servo control system detects the track over which the read/write head is positioned and controls movement of the head relative to the track.
There are a variety of methods for providing servo information to a disk servo control system. In a method referred to as the dedicated servo method, the entire surface of one disk is provided with servo information. A servo magnetic head is positioned over the dedicated servo disk surface in a fixed relationship relative to one or more data read/write heads positioned over data disk surfaces. The position of the servo head is used to indicate the position of the data read/write heads. The dedicated servo method is most often used with multiple disk systems, because a dedicated servo system for a single disk application would use one-half of the available disk surface area for servo information and therefore would not be especially efficient.
Another method of providing servo information is known as the sector servo method. In the sector servo method, each disk surface includes servo information and binary data. The tracks on a disk surface are divided into radial sectors having a short servo information area followed by a data area. The servo information area may include a sector marker, which indicates to the read/write head that servo information immediately follows in the track, track identification data, a high-frequency servo burst pattern, a synchronization field providing the servo synchronization process, and a pad field used for "padding" to allow for disk rotational timing error. The sector servo method is more efficient than the dedicated servo method for low profile disk drives with fewer disks in the configuration, because a single read/write head can be used to obtain the servo information and to read and record data from the disk and also because less of the disk surface area is used for servo information. As users demand greater storage capacities from low profile disk systems, manufacturers provide less and less disk area for servo information, by decreasing sector length and track width. To obtain the same amount of servo information in less disk area, the servo information must be recorded at higher and higher frequencies. The higher frequencies increase the difficulty of writing and reading the servo information.
In both the dedicated servo and sector servo methods, an analog position error signal (PES) is produced as the servo pattern is read from the disk and is used to generate a corrective input signal to the read/write head positioning servo. The remaining description assumes a sector servo system, but it will be clear to those skilled in the art how the description can be applied to dedicated servo systems. The servo pattern flux reversals are distributed about each track centerline and, when read from the disk and demodulated, provide a PES whose amplitude depends on the location and orientation of flux reversals in the track located beneath the read/write head. The PES provides an indication of the direction and extent of read/write head movement required to maintain the head centered about the track.
More particularly, the PES is produced, or demodulated, from the flux transitions by determining the amplitude difference of information read from each side of the track centerline. The resulting PES indicates the deviation of the read/write head from the track centerline. If the amplitude difference in information from both sides of the centerline is zero, then it is assumed that the read/write head is positioned exactly over the track centerline. A positive amplitude difference in the information indicates that the head is off center in one direction and a negative amplitude difference in the information indicates that the head is off center in the opposite direction.
The majority of conventional disk drive systems demodulate the PES using analog methods. As the servo pattern flux transitions on either side of a track centerline pass by the magnetic read/write head, the head produces an amplitude-varying analog signal that is sent to a preamplifier. An automatic gain control circuit typically receives the preamplified signal and produces a signal with reduced dynamic range, which makes the signal easier to process and can thereby reduce errors. Analog demodulation techniques provide a position error signal (PES) that indicates the position of the read/write head relative to the track centerline. The PES can be provided to a servo controller to control the disk arm servo and keep the read/write head centered about the track. The PES also can be provided to an analog-to-digital converter to produce a digital position error signal, which is then used to control the disk arm servo.
It also is known to demodulate the PES using digital signal processing techniques. See, for example, U.S. Pat. No. 5,089,757 to Wilson entitled "Synchronous Digital Detection of Position Error Signal." Digital techniques permit the sharing of components such as preamplification, automatic gain control, and analog-to-digital conversion elements between the PES processing system and the binary data processing system, thereby simplifying overall servo control circuit construction. In addition, digital demodulation permits the use of relatively sophisticated signal processing techniques that are not easily implemented with analog demodulators. These techniques can be used, for example, to remove spurious signal artifacts resulting from other system components or from the analog-to-digital conversion process itself.
Many digital PES demodulator systems are of the synchronous type, in which sampling of the servo information from the disk and analog-to-digital conversion of the signal are synchronized with the storage device system clock. Such synchronous demodulation systems require a synchronization field in the servo information areas of the disk and use a phase-lock-loop (PLL) to control the servo information sampling and analog-to-digital conversion. Unfortunately, the synchronization field reduces the disk area available for recording of data. In addition, the PLL can introduce processing errors that can require additional compensating circuit components, complicating the design and construction of the digital PES demodulator.
The desire for increased storage capacity, resulting in what are commonly referred to as high density disk drive systems, also has resulted in new read/write head technologies. For example, magneto-resistive (MR) read/write heads are becoming more common because they permit reading of data at relatively high frequencies even with lower disk rotational velocity. The higher frequencies permit servo information and binary data to take up less disk space, increasing disk capacity. Unfortunately, the nonlinear characteristics of MR read/write heads result in strong second-order harmonics in the read signal that can introduce extra errors in the resulting PES, which can cause mistracking of the read/write head.
A conventional servo pattern typically extends across the full width of the data tracks in a staggered fashion across the disk surface and is recorded by a magnetic head that extends across only a portion of the track. Therefore, the servo pattern flux transitions typically are recorded by multiple passes of the magnetic head relative to the servo information area. With each pass, a different portion of the servo pattern is recorded until the entire pattern is completed. See, for example, the article "Quad-Burst PES System for Disk File Servo" by W. A. Herrington and F. E. Mueller, published in IBM Technical Disclosure Bulletin Vol. 21, No. 2 (July 1978) at pages 804-805.
In particular, FIG. 1 shows a conventional servo pattern 10 recorded in tracks across a disk. Only four tracks 12, 14, 16, 18 are illustrated for simplicity. The servo pattern is comprised of bursts of an even number of sequential flux transitions, represented as vertical bars 20, that are recorded at a predetermined transition frequency in a group of four bursts staggered across the disk on each sector. For reasons of linearity known to those skilled in the art, the magnetic head that records the flux transitions in the tracks records a flux orientation of no more than one-half track width at a time. Each flux transition 20 extends across the entire width of a track and therefore requires multiple passes of the head. Thus, two flux transitions 20a and 20b are aligned in the disk radial direction to form a single flux transition. The multiple passes can result in mis-alignment of flux transitions from adjacent passes. Even small misalignnments can produce phase errors when the recorded servo information is read at a later time. It would be advantageous if a simplified servo pattern could be used that would reduce the likelihood of misalignment between flux transition passes. Moreover, it also would be advantageous if a servo pattern could be more easily recorded in the reduced track widths that are becoming more commonplace.
From the discussion above, it should be apparent that there is a need for a digital PES demodulator with reduced overall circuit complexity that makes use of effective digital signal processing techniques to accommodate high frequency servo patterns and reduce head mistracking. It also should be apparent that there is a need for simplified servo patterns that can be accommodated in narrow data tracks and can be used with digital PES demodulators, and that reduce the likelihood of misalignment errors. The present invention fulfills these needs.